Investors have a new tool at their disposal when it comes to determining whether they’re getting what they pay for when they invest in actively managed funds. In April, following an investigation by the New York attorney general’s office, 13 mutual fund firms, including T. Rowe Price and Vanguard, agreed to disclose to individual investors a previously unpublished measure called active share.

Active share measures the extent to which a mutual fund’s holdings deviate from a market index. An index fund would have 0% active share because its holdings completely align with those of the benchmark. In theory, the measure, when combined with expenses, allows the investor to determine how much active management he or she is paying for. If an actively managed fund holds 90% of an index, the investor may be better off with a low-cost index fund.

It’s important to remember, however, that active share has no correlation with performance, says Jeff Ptak, global head of manager research at Morningstar. Rather, before you buy a fund, try to determine whether active management is worth paying for, he says. “Investors can determine whether a fund veers from its benchmark or stays at home. If it’s the latter, then investors should begin to research how that manager adds value to the fund.”